1. Field of the Invention
This invention relates to a film for film capacitors.
2. Description of the Related Art
Presently, a dielectric film for film capacitors is made of a resin selected from 4 types of resin compositions such as a polypropylene resin (PP resin), a polyethylene terephthalate resin (PET resin), a polyphenylsulfide resin (PPS resin) and a polyethylene naphthalate resin (PEN resin). A capacitor is constructed by vacuum depositing metal films onto both sides of the dielectric film.
However, a PP resin film is used at 105 degrees C. or less and a PET resin film is used at 125 degrees C. or less, because heat resistant properties of the PP resin and the PET resin are poor. On the other hand, a capacitor used for an inverter requires heat resistance of 150 degrees C. or more, due to the widespread use of hybrid cars. Accordingly, in order to apply the PP resin film or the PET resin film to the capacitor of hybrid cars, the following 2 countermeasures must be employed: installing a large cooling unit without any regard to weight reduction; installing a capacitor on a driver seat side, etc., which is far from the engine room, or a heat source. Consequently, weight reduction and cost reduction are the problems to be solved, with regard to the capacitor of hybrid cars.
Though a PPS resin has good heat resistance and a PPS resin film for film capacitors can be used at 160 degrees C. or less, it has limited range of use because of its low dielectric breakdown voltage and low voltage proof. Further, though PEN resin has also good heat resistance and a PEN resin film for film capacitors can be used at 160 degrees C. or less, it has limited range of use because of its high dielectric loss and its large temperature dependence of dielectric tangent.
On the other hand, a polyetherimide resin (PEI resin) film draws attention as a film for film capacitors as disclosed in Japanese Laid Open Patent Application Publication No. 2007-300126. A film composed of a PEI resin is suitable for a film for film capacitors because of its high heat resistance resulting from its high glass transition point of 200 degrees C. or more, its high dielectric breakdown voltage and high voltage proof, and its small frequency dependence and temperature dependence of dielectric tangent.
A thin film composed of a thermoplastic resin used as a film for film capacitors lacks in lubricity (or slidability) and so sometimes causes troubles such as interruption of winding operation or slitting operation of the film, appearance of wrinkles on the film, and sticking of the film to guide rolls, etc., during manufacturing operation of the film. Further, assembling of a capacitor from the film is sometimes interrupted by blocking of the film or breakage of the film during uncoiling the film. For this reason, slidability of the film should be improved, in order to use the thermoplastic resin film as a film for film capacitors.
Each of the PP resin, the PET resin, the PPS resin and the PEN resin is a thermoplastic crystalline resin. A film composed of any of the resins is manufactured by so called “a biaxially stretching method”. In the biaxially stretching method, a melt resin is extruded through a T-die, the extrusion is cooled and solidified in a casting unit and it is stretched through a longitudinally stretching machine and a transversely stretching machine in turn, and then a biaxially stretched film is rolled up onto a winding tube. In a thin film manufactured through the biaxially stretching method, even if an emboss patterned surface of the cooling roll is transferred onto the film surface during cooling, the transferred embossed pattern on the film shall be finished to a mirror surface by the following stretching processes. For this reason, in order to increase the slidability of the film composed of any of these thermoplastic crystalline resins, a surface roughening method in which different phase particles or fillers are dispersed in the resin is employed, as disclosed in Japanese Laid Open Paten Application Publication No. 2007-308604 and Japanese Laid Open Paten Application Publication No. 2009-132874.
Japanese Laid Open Patent Application Publication No. 2007-308604 discloses that in order to provide a biaxially stretched polypropylene film easily slidable and excellent in device winding processability during capacitor assembling, it is effective to limit beta phase fraction in the film in a range of 5-25%, and to form a fine rough surface to the film so that the proportion of the area below the surface roughness curve higher than 0.1 micron beyond the average base line falls in a range of 15-30% of the total area below the surface roughness curve beyond the average base line.
Further, Japanese Laid Open Patent Application Publication No. 2009-32874 discloses a biaxially-oriented polyarylenesulfide film made of a thermoplastic resin composed of a polyarylenesulfide containing a PPS resin of 80 mole % or more, and other thermoplastic resin A, in which the thermoplastic resin A forms dispersion phase and the center line average roughness (Ra) of the film is in a range of 20-200 nm, and the maximum height Rmax of the film is 1000 nm or less. It is reported that the film is easily slidable, does not get wrinkled in film processing and shows no problem in slitting operation and capacitor assembling.
As mentioned, a crystalline thermoplastic resin film is manufactured by extruding a melt film through a T-die, cooling and solidifying the extruded melt film in a casting unit and then biaxially stretching the solidified film. The biaxially stretching process can not be applied to a PEI resin film manufacturing, because the PEI resin is one of amorphous thermoplastic resins and so shows poor ductility. For this reason, a final thickness of the PEI resin film is given at the cooling and solidifying process of a melt film extruded through a T-die, on a cooling roll. Japanese Laid Open Patent Application Publication No. 1996-20060 discloses that a PEI resin film having good transparency and easy slidability applicable to various usages is obtained by forming embossed surface of surface roughness in a range of 0.1-0.5 micron onto the film surface, by melt extruding a PEI resin on to a cast roll having embossed surface.